1. Field of the Invention
This invention relates to a thermal transfer recording apparatus which records using ink sheets, and to a facsimile apparatus equipped with such a recording apparatus and to a method thereof.
Apart from a facsimile apparatus, the thermal transfer recording apparatus of this invention is applicable to office equipment such as an electronic typewriter, word processor, copying machine, and computer, and further, to communication equipment.
2. Description of the Related Art
There are various different types of thermal transfer printers including the type of printer using so-called one-time ink sheets, with which the ink is completely transferred from the ink sheet onto the recording paper at one printing, and a printer using so-called multi-time ink sheets that allow recording to be effected a plurality of times (N times) at any given position on the sheet, with the recording paper and the ink sheet being fed at different speeds. Thermal transfer printers of the latter type have higher operating costs as compared with the former; in view of this, nowadays, such printers are generally used as the recording unit in a facsimile apparatus or the like. Further, U.S. Pat. No. 4,910,602 shows a thermal transfer printer in which an ink sheet is contained in a case which can be detachably attached to the recording apparatus body, the ink sheet being loaded and unloaded along with this case. With this type of thermal transfer printer, the multi-time ink sheet is so constructed that it is taken up by driving a take-up roller by means of an ink-sheet take-up motor. The take-up torque for driving this take-up roller is set to a constant value for all the take-up stages of an ink sheet, as in conventional thermal transfer printers. Thus, even in a stage where most of the ink sheet has been wound onto the take-up roller, that is, even when the ink sheet is near its end, the same torque is applied to drive the take-up roll as is used in initial stages.
A problem with this type of ink-sheet take-up operation is that in the initial stages of use of the ink sheet, the ink sheet advances too far in the feeding direction, and so it has to be rewound a bit after each feed; as a result of repeating this useless movement, a noise is generated, or an extra load is applied to the ink sheet.
After assiduous study of this behavior, the present inventors found that the above problem was due to the fact that the same torque was used to drive the take-up roller in both the initial and final take-up stages. This may be explained as follows: In the initial stages of use, the ink-sheet supply roller has a relatively large diameter, whereas the diameter of the take-up roller is relatively small, which means the driving torque required to rotate the take-up roller is relatively small. In the later stages of use of the ink sheet, in contrast, the take-up roller has a relatively large diameter, and its weight is also larger than in the initial stages; as a result, rotation of the take-up roller requires a larger driving force, which means the take-up torque supplied by the take-up motor must be increased as the ink sheet approaches the final stages of its use. This difference in requisite torque between the initial and final stages of use, attributable to the change in the diameter of the take-up roller, is quite large due to the fact that a back tension in the direction of the supply roller is being applied to the ink sheet in order to eliminate any sag thereof. Furthermore, it should also be remembered that the thermal head is being pressed against the platen roller with a large force. Thus, if, as in the above example, the torque applied to the take-up roller is kept constant, an excessively large torque must be applied to the take-up roller in the initial stages of use of the ink sheet, and this will cause, for example, an overshoot of the rotating shaft of the take-up motor, making the ink-sheet move unnecessarily, thereby generating a noise, applying an extra load on the ink sheet, or giving rise to a loss in energy (wasteful power consumption) of the take-up motor.